PLATE TECTONICS

Scientists believe that the Earth’s outer crust is
made up of about huge fragments, called tectonic plates, that fit together like
a cracked eggshell. According to the theory of plate tectonics, devised in the
1970s, these plates ride like rafts on the softer, red-hot rock below and very
move slowly over the globe, carrying the continents with them. Past
arrangements of tectonic plates created one vast SUPERCONTINENT.

FRACTURED CRUST

Earth’s crust is a giant jigsaw of seven enormous plates and
about twelve smaller ones. Many scientists believe plate movement is driven by
slow-churning currents deep in the mantle beneath. As the plates drift, they
converge (move towards each other) and collide, or grind past one another at
transform margins, or diverge (pull apart).

PLATE BOUNDARIES

The edges of the plates that make up the lithosphere are called
boundaries or margins. New crust is mainly created at plate boundaries in
mid-ocean, where the SEA-FLOOR IS SPREADING. Older crust is
destroyed near the edges of oceans, where plates collide and one subducts
(dives) below the other and melts. This causes the plates to move very slowly
over the softer asthenosphere, below.

SUPERCONTINENT

The shapes of continents such as eastern South America
and western Africa would fit neatly if pushed together. The discovery of
matching fossils and rock layers on land separated by wide oceans provided
further evidence that landmasses were once united. Scientists call this
supercontinent Pangaea. The slow movement of Earth’s plates caused
Pangaea to split apart.

PANGAEA

Some 300 million years ago, plate movement drove Earth’s
landmasses together to form Pangaea (All-Earth). This was surrounded by the
vast ocean Panthalassa. About 100 million years later Pangaea began to break
up.

MOVING CONTINENTS

An arm of the Tethys Sea, an ancient ocean, opened to split
Pangaea in two. To the north lay Europe, North America, Greenland, and Asia,
with South America, Africa, India, Australia, and Antarctica to the south.

CONTINENTS TODAY

As plate movement continued, these large fragments split into
smaller continents, which slowly came to their present positions. They continue
to move at a rate of a few centimetres per year.

BIOGRAPHY: ALFRED WEGENER German, 1880-1930

Climate expert and geophysicist Alfred Wegener pioneered the
theory of continental drift in 1915. He became convinced that the continents
were once joined, and put forward the idea of Pangaea. On the Arctic island of
Spitzbergen, Wegener found fossils of tropical ferns, which suggested that the
island had once lain in the tropics. His ideas were not taken seriously until
the 1960s.

SEA-FLOOR SPREADING

Mountain chains, longer and mightier than any on land,
run down the centre of the oceans. At these mid-ocean ridges, where tectonic
plates diverge, molten magma erupts to bridge the gap. Rock samples taken from
the Atlantic floor in the 1960s showed that the youngest rocks lay in the
centre of the ridges, with older rocks to either side. As the new rock forms,
older rock is pushed aside, and the sea floor widens, or
spreads.